How do you price a letter from Lee?

I see no pattern in these sales, unless price correlates to buyer familiarity with the surrounding history.

Since no one has the least inkling about the Sherman (right) vs. Lee campaign of late 1861, Lee letters from this period will fetch lower prices (and prompt much head scratching).

The feebleness of ACW history over the last 50 years is thereby felt in the antiquities market...